A vacuum countergravity casting process using a gas permeable mold is described in such prior art patents as the Chandley et al U.S. Pat. Nos. 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) secured together, sealing a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, submerging the bottom side of the drag in ar underlying pool of molten metal and evacuating the vacuum chamber to draw the molten metal through one or more ingate passages in the drag and into one or more mold cavities formed between the cope and the drag.
In practicing that vacuum countergravity casting process, the molten metal pool typically is contained in a melt-holding vessel over an extended time period (e.g., about 5-10 minutes) as required to countergravity cast a plurality of molds in succession from the molten metal pool. Attempts by the inventor to hold a melt, such as a grey iron or a nodular iron melt, over such an extended time period have met with difficulties in maintaining the proper melt casting temperature. The particular melt-holding vessel used in these attempts included a steel support shell having an inner, solid refractory lining defining a cylindrical melt-holding chamber. A coreless induction coil disposed below the melt-holding vessel was continuously energized to inductively heat the melt in an attempt to maintain its temperature within the desired range for casting over the necessary extended time period. However, as a result of unexpectedly high heat loss from the melt by conduction through the refractory side wall of the vessel, the melt-holding vessel was incapable of maintaining the temperature of the grey iron or nodular iron melt within the desired range for the time period required to cast a plurality of molds in succession from the pool, even when the induction coil was energized continuously at its maximum power limit or rating (e.g., 840 kilowatts).
It is an object of the present invention to provide an improved melt-holding vessel having means for substantially reducing heat loss from the melt by conduction through the refractory side wall of the vessel to enable the temperature of the melt to be maintained within the desired range for casting with a reduced level of energy input to the melt.
It is another object of the invention to provide an improved method of casting a melt from a melt-holding vessel involving reducing conductive heat loss from the melt through the vessel ride wall to such an extent that the melt temperature can be maintained within the desired range for casting one or more molds over an extended time period.